Abstrakt
Ni2MnGa is a good model system of the shape memory alloy family. These materials are frequently studied because of their possible application as micropumps, sensors or actuators.
Shape memory effect is connected to the martensitic transformation and to the magnetically induced reorientation (MIR). Therefore, the study of these materials under various external conditions is the key to understand the martensitic transformation, which is still not fully described.
Previous studies showed that critical temperatures of the Ni2MnGa related compounds are strongly dependent on their composition. For example, Ni2MnGa has the martensitic transformation around 200 K, but the alloy with the slightly different composition Ni50Mn28Ga22 has the martensitic transformation at 338 K.
According to abovementioned facts we decided to study properties of Ni2MnGa single crystals with respect to indium doping (galium is replaced). As the MIR occurs by twin boundary motion, a single crystal is the basic prerequisite for any significant effect as the presence of grain boundaries can block the motion of twin boundaries.
The single crystals were prepared by Bridgman method in the induction furnace (growing speed 0.5 mm/h). The material Ni2MnGa1-xInx was previously studied in the form of polycrystals but the results obtained on single crystals apparently differ as it is shown in this work.
Electrical resistivity, magnetisation and specific heat measurements were performed to obtain the critical temperatures. All obtained critical temperatures (Curie temperatures, temperatures of martensitic and premartensitic transformation) decrease with the increasing indium content.
Contrary to the results previously published on polycrystals, the decreasing tendency is much faster - the polycrystalline samples exhibited martensitic transformation even in the sample with 15% of indium, but we did not observed any martensitic transformation on single crystals with the indium content above 10%.